Charge modified microporous membrane

ABSTRACT

A cationic charge modified microporous membrane is provided. The membrane comprises a hydrophilic organic polymeric microporous membrane having bonded thereto, through a cross-linking agent, a charge modifying amount of a cationic charge modifying agent. The charge modifying agent is an aliphatic amine or polyamine, preferably tetraethylene pentamine, and the cross-linking agent is an aliphatic polyepoxide having a molecular weight of less than about 500, preferably 1, 4 butanediol diglycidyl ether. The microporous membrane is preferably nylon. The process for applying the charge modifying agent to the membrane, preferably comprises contacting the membrane with an aqueous solution of the charge modifying agent and then contacting the membrane with a substantially aqueous solution of the cross-linking agent. The preferred process uses high purity water, i.e. water having no significant ionic content, to produce a membrane suitable for use in filtering ultra pure (18 megohm-cm resistivity) water for use in producing electronic devices. Such a membrane exhibits an advantageously low &#34;flush-out&#34; time compared to known cationic charge modified microporous membranes and unmodified microporous membranes.

This is a continuation of Ser. No. 268,573, filed 5-24-81, now U.S. Pat.No. 4,473,475.

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

This invention relates to microporous membranes and more particularly tocationic charge modified microporous membranes suitable for thefiltration of high purity water as used in the electronics industry.

2. PRIOR ART

Microporous membranes are well known in the art. For example, U.S. Pat.No. 3,876,738 to Marinaccio et al (1975) describes a process forpreparing a microporous membrane, for example, by quenching a solutionof a film forming polymer in a non-solvent system for the polymer.European Patent Application No. 0 005 536 to Pall (1979) describes asimilar process to Marinaccio et al.

Other processes for producing microporous membranes are described forexample, in the following patents:

U.S. Pat. No. 2,783,894 to Lovell et al;

U.S. Pat. No. 3,642,668 to Bailey et al;

U.S. Pat. No. 4,203,847 to Grandine, II;

U.S. Pat. No. 4,203,848 to Grandine, II; and

U.S. Pat. No. 4,247,498 to Castro.

Attempts have also been made to charge modify microporous membranes. Forexample, U.S. Pat. No. 4,125,462 to Latty (1978) describes a coatedmembrane having a layer or coating of a cationic polyelectrolyte,preferably poly(vinylimidazoline) in the bisulfate form. Other types ofcharge modified membranes are described in, for example, U.S. Pat. No.3,556,992 to Massucco; U.S. Pat. No. 3,556,305 to Shorr; U.S. Pat. No.3,808,305 to Gregor; and U.S. Pat. No. 4,250,029 to Kiset et al.

In the assignee's U.S. Ser. No. 201,366 filed Oct. 27, 1980 byOstreicher et al now abandoned, and copending U.S. Ser. No. 314,307filed on Oct. 23, 1981, now U.S. Pat. No. 4,473,474, which is acontinuation-in-part of U.S. Ser. No. 201,366, now abandoned, aparticularly preferred charge modified microporous membrane isdescribed. The membrane, preferably nylon, comprises a multiplicity ofcationic charge sites on the internal pore surfaces. The charge sitesare provided by a cationic charge modifying resin, in particular apolyamido-polyamine epichlorohydrin resin, bonded to the membranestructure. The membrane is further provided with a cross-linking agentfor the charge modifying resin which is effective in retaining the resinon the membrane. A preferred cross-linking agent is a polyamine, e.g.tetraethylene pentamine. Such a membrane is sold under the TrademarkZETAPOR BY AMF Cuno, Meriden, Conn.

The primary advantage of a charge modified microporous membrane is theexclusion of particulate on the basis of charge as well as size; thus,for example, viruses can be removed from a fluid without having to go toan ultrafiltration membrane with its associated high pressures.

In the production of solid state electronic devices, e.g. computerchips, it is critical that any water utilized in the production processbe of the highest possible purity in terms of freedom from bothdissolved and undissolved contaminants. Normally, filtered 18 megohm-cmresistivity water is used in such applications. Such water is generallyproduced by activated carbon and ion exchange deep bed treatments (toremove dissolved organic and inorganic contaminants) followed byfiltration (to remove small particulate and undissolved contaminants).Given the practical cost and particle size removal limitations ofconventional mechanical barrier type filters, vis-a-vis charge modifiedfilters, the present filtration systems are not completely satisfactoryin terms of removal of colloidal contaminants, waterborne bacteria, etc.Additionally, without exception, all of the known filtration mediaseriously reduce the resistivity of the deionized water being filteredby introducing a significant amount of inorganic ionic contamination tothe water. While conventional polymeric membrane filters which are notcharge modified suffer from this defect to some degree, they tend toflush out very rapidly, e.g. within 5 to 30 minutes. Such a flush outprocedure is a normal part of the water system start up after changing afilter. Such mechanical barrier type membrane filters remove smallparticulate and undissolved contaminants, by providing a sufficientlysmall effective pore dimension to remove, by mechanical straining, theundesired contaminants. Such filter structure, in the form ofmicroporous membranes of, for example, 0.1 micrometer rating or less,may be readily prepared. Such membranes tend to flush out very rapidly,however, the flow rates exhibited by such structures at conventionalpressure drops are prohibitively limited. Increasing the pressure dropthrough the membrane to provide the desired flow rate is generally notfeasible, even with costly replacement or modification of existingpumping equipment. This is due to the fact that pressure drop is afunction of the fourth power of flow rate.

While it would appear advantageous to use a cationically charge modifiedfilter media, e.g. a microporous membrane, to provide for the economicaland effective removal of undissolved particulate contaminants, currentstate of the art media exhibit certain characteristics that preventtheir use in such applications. Currently available charge modifiedmicroporous media, for example, the aforementioned ZETAPOR cationicallymodified nylon membrane prepared in accordance with the aforementionedU.S. Ser. No. 201,366, exhibit an extremely slow flush outcharacteristic, i.e. up to several hours. This characteristic hasprevented the use of such charge modified media in such high purityelectronic water systems.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of this invention to provide a novel cationic chargemodified microporous membrane.

It is a further object of this invention to provide a process forcationically charge modifying a hydrophilic organic polymericmicroporous membrane.

It is a further object of this invention to provide a process for thefiltration of fluid, and in particular the filtration of high puritywater used in the electronics industry.

It is a further object of this invention to provide a cationic chargemodified microporous membrane which has rapid flush out characteristicssuitable for the electronics industry.

These and other objects of this invention are attained by a novelcationic charge modified microporous membrane. The membrane comprises ahydrophilic organic polymeric microporous membrane and a chargemodifying amount of a cationic charge modifying agent bonded to themicroporous membrane structure through an aliphatic polyepoxidecross-linking agent having a molecular weight of less than about 500.

The charge modifying agent is selected from the group consisting of:

(i) aliphatic amines having at least one primary amine or at last twosecondary amines; and

(ii) aliphatic amines having at least one secondary amine and a carboxylor hydroxyl substituent.

The invention is further directed to a process for cationically chargemodifying a hydrophilic organic polymeric microporous membrane byapplying to the membrane the aforesaid charge modifying agent bonded tothe membrane through the aforesaid cross-linking agent.

The preferred microporous membrane is nylon, the preferred polyamine istetraethylene pentamine and the preferred polyepoxide is 1,4-butanedioldiglycidyl ether.

Preferably, the process for charge modifying the microporous membranecomprises contacting the membrane with an aqueous solution of thepolyamine and subsequently contacting the membrane with a substantiallyaqueous solution of the polyepoxide. The aqueous solutions arepreferably prepared from high purity water having no significant ioniccontent.

The membrane may be used for filtration of fluids, but is particularlysuitable for the filtration of high purity water used for producingelectronic devices, e.g. computer chips.

DETAILED DESCRIPTION OF THE INVENTION

The cationic charge modified microporous membrane of this invention isproduced from a hydrophilic organic polymeric microporous membrane. Suchmembranes are well known in the art. A preferred microporous membrane isone produced from nylon. The preferred membrane substrates are producedby the method disclosed in U.S. Pat. No. 3,876,738 to Marinaccio et al.Another method of producing such membranes is described in EuropeanPatent Application No. 0 005 536 to Pall. The entire disclosures of bothof these references are incorporated herein by reference.

Additionally, any of the hydrophilic microporous membranes commerciallyavailable, for example, Pall Corp.'s ULTIPOR NM, having characteristicspotentially suitable for fine filtration of fluids, particularly highpurity water are suitable for treatment in accordance with thisinvention.

Preferably, the membrane is prepared from a hydrophilic synthetic linearrelatively high molecular weight polymeric substance, most preferably apolyamide such as polyhexamethylene adipamide (Nylon 66). This nylonpolymer may be converted into membrane form without humectants,plasticizers or similar additives; is wettable by aqueous liquids; andoffers the flexibility, stability and chemical properties suitable foruse in filtration. While other polyamides, e.g. Nylon 6, are alsosuitable, for ease of conversion into the membrane form, chemicalresistance, strength and other considerations, Nylon 66 is preferred.

The conversion of the polymer into a microporous membrane may beconducted by methods described in the art, but in particular, the methoddescribed in U.S. Pat. No. 3,876,738 to Marinaccio et al is preferred.The Marinaccio et al process develops a unique fine internalmicrostructure through the quench technique described therein, offeringa superior substrate for filtration.

These preferred nylon membranes, are characterized by an isotropicstructure, having a high effective surface area and a fine internalmicrostructure of controlled pore dimensions with narrow pore sizedistribution and adequate pore volume. Thus, a representative 0.22micrometer rated Nylon 66 membrane (polyhexamethylene adipamide)exhibits an initial bubble point of about 45 to 50 psid., a foam allover point of about 50 to 55 psid, provides a flow of from 70 to 80ml/min of water at 5 psid (47 mm. diameter discs), has a surface area(BET, nitrogen absorption) of about 13 m² /g and a thickness of about4.5 to 4.75 mils.

By the use of the term "hydrophilic" in describing the microporousmembrane, it is meant a membrane which absorbs or adsorbs water.Generally, such hydrophilicity is produced by hydroxyl (OH⁻), carboxyl(--COOH) and/or amino (--NH²) substituents on the surface of themembrane. Such substituents assist in the absorption or adsorption ofthe water onto the membrane. Such hydrophilicity of the membrane, i.e.,presence of hydroxyl, carboxyl and amino substituents, is a necessaryelement of this invention to provide the adequate bonding of the chargemodifying agent through the cross-linking agent to the microporousmembrane.

The charge modifying agent used in this invention is selected from thegroup consisting of:

(i) aliphatic amines having at least one primary amine or at least twosecondary amines; and

(ii) aliphatic amines having at least one secondary amine and a carboxylor hydroxyl substituent.

Preferably, the charge modifying agent is a polyamine having theformula: ##STR1## wherein R₁ and R₂ are alkyl of 1 to 4 carbon atoms andx is an integer from 0 to 4. Preferably, R₁ and R₂ are both ethyl.

Preferred polyamines are:

Ethylene Diamine: H₂ N--(CH₂)₂ --NH₂

Diethylenetriamine: H₂ N--(CH₂)₂ --NH--(CH₂)₂ --NH₂

Triethylenetetramine: H₂ N--(CH₂ --CH₂ --NH)₂ --CH₂ --CH₂ --NH₂

Tetraethylenepentamine: H₂ N--(CH₂ --CH₂ --NH)₃ --CH₂ --CH₂ --NH₂

The highly preferred polyamine is tetraethylene pentamine.

Alternatively, aliphatic amines used in this invention may have at leastone secondary amine and a carboxyl or hydroxyl substituent. Exemplary ofsuch aliphatic amines are gamma-amino-butyric acid (H₂ NCH₂ CH₂ CH₂COOH) and 2-aminoethanol (H₂ NCH₂ CH₂ OH).

The charge modifying agent is bonded to the microporous membranestructure through an aliphatic polyepoxide cross-linking agent having amolecular weight of less than about 500. Preferably, the polyepoxide isa di- or tri-epoxide having a molecular weight of from about 146 toabout 300. Such polyepoxides have viscosities (undiluted) of less thanabout 200 centipoise at 25 C.

Highly preferred polyepoxides have the formula: ##STR2## wherein R is analkyl of 1 to 6 carbon atoms and n is from 2 to 3. The limitation thatthe number of carbon atoms in the non-epoxide portion --(R)-- be lessthan 6 is so that the polyepoxide will be soluble in water orethanol-water mixtures, e.g. up to 20% ethanol. While higher carboncontent materials are functionally suitable, their application wouldinvolve the use of polar organic solvents with resulting problems intoxicity flammability and vapor emissions.

It is preferred that certain diglycidyl ethers of aliphatic diols beused as the polyepoxide cross-linking agents. These are generallyproduced by reacting an aliphatic diol with epichlorohydrin underappropriate reaction conditions. The reaction may be represented asfollows: ##STR3##

For example:

When R is (CH₂)₂,HO--R--OH is 1,2-ethanediol

When R is (CH₂)₃,HO--R--OH is 1,3-propanediol

When R is (CH₂)₄,HO--R--OH is 1,4-butanediol

The diglycidyl ether of 1,4-butanediol, the preferred polyepoxidecross-linking agent, is commercially available from Ciba-Geigy, Inc. asRD-2 and from Celanese Corp. as Epi-Rez 5022.

Other higher carbon diglycidyl ethers may be used as the polyepoxidecross-linking agent, for example when R is (CH₂)₅ the 1,5-pentanedioldiglycidyl ether is produced. As stated previously, however, theappropriate polar organic solvents must be used for diluting suchpolyepoxides.

Triglycidyl ethers, i.e. tri-epoxides may also be utilized as thepolyepoxide cross-linking agent. For example, the triglycidyl ether ofglycerol may be utilized. This tri-epoxide may be produced by thefollowing reaction: ##STR4## The triglycidyl ether of glycerol isavailable from Shell, Inc. as Epon 812 and from Celanese Corp. asEpi-Rez 5048.

The amount of cationic charge modifying agent utilized is an amountsufficient to enhance the electropositive capture potential of themicroporous membrane. Such an amount is highly dependent on the specificcharge modifying agent utilized.

Broadly, the process of this invention is directed to cationicallycharge modifying a hydrophilic organic polymeric microporous membrane,e.g. nylon. The process comprises applying to the membrane a chargemodifying amount of the cationic charge modifying agent bonded to themembrane structure through the polyepoxide. Preferably, the processcomprises (a) contacting the membrane with an aqueous solution of thecationic charge modifying agent and (b) contacting the membrane with anaqueous solution of the polyepoxide cross-linking agent. The contactingsteps may be performed in any order, i.e. step (a) prior to step (b) orvice versa. Such contacting steps also include contacting the membranewith an aqueous solution of a mixture of the charge modifying agent andthe polyepoxide cross-linking agent. It is preferred, however, foroptimum (minimum) flushout times to first contact the membrane with theaqueous solution of the cationic charge modifying agent and thensubsequently contact the so treated membrane with the aqueous solutionof the polyepoxide cross-linking agent. It is preferred, however, formaximizing charge modification to contact the membrane with an aqueoussolution of a mixture of the charge modifying agent and the polyepoxidecross-linking agent.

In order to provide the charge modifying amount of cationic chargemodifying agent for the membrane, it is preferred that the aqueoussolution of charge modifying agent that the membrane is contacted withcontain at least about 0.25% charge modifying agent, by weight of thesolution, preferably a polyamine. The maximum amount of charge modifyingagent in the aqueous solution is limited by economic and solubilitylimitations. For example, an excess of charge modifying agent which isnot bonded to the microporous membrane will not be economically utilizedand will constitute an extractive from the membrane. It has been foundthat the amount of charge modifying agent in the aqueous solution shouldprobably not exceed about 1% by weight of the solution.

The amount of polyepoxide used in the aqueous solution is highlydependent on the specific polyepoxide and the amount and type chargemodifying agent used, and the cross-linking mechanism between thesecompounds to provide the bonding of the charge modifying agent to themicroporous membrane. For general guidance however, it has been foundthat a weight ratio of polyepoxide to charge modifying agent from about2:1 to about 10:1, preferably from about 3:1 to about 5:1, in theaqueous solution(s) contacted with the membrane, is generally sufficientto provide the bonding of the cationic charge modifying agent to themembrane through the polyepoxide cross-linking agent. It has been foundthat if the aqueous solution containing the polyepoxide contains atleast about 1% polyepoxide by weight of the solution, up to a maximum ofabout 4% polyepoxide by weight of the solution when used in conjunctionwith the aforementioned aqueous solution of charge modifying agent, thatadequate bonding of the charge modifying agent to the microporousmembrane is obtained.

Both the charge modifying agent and the polyepoxide cross-linking agentmay be contacted with the membrane by dipping the membrane in theaqueous solution(s) of these compounds for a period of time sufficientto effect the desired degree of pick-up. Alternatively, the chargemodifying agent and/or cross-linking agent may be applied by spraying orcontacting a wick or roll along the surface of the microporous membrane.

In the highly preferred process for producing a microporous membrane foruse in filtering high purity water for the electrical industry, theaqueous solution of charge modifying agent and polyepoxide are producedwith water having no significant ionic content. Such water should have aresistivity of at least about 250,000 ohm-cm. Production of the membranewith such water is desirable so that the membrane is not ionicallycontaminated.

Although applicants do not wish to be bound by the following theory, itis believed that in bonding the charge modifying agent to themicroporous membrane the polyepoxide not only acts as a cross-linkingcoupling agent but also functions to a certain degree to enhance thecationic charge modification produced.

It is theorized that the epoxide groups on the polyepoxide cross-linkingagent enter into an addition type reaction with the hydroxyl, carboxyland primary and secondary amines, which are on the hydrophilicmicroporous membrane and the cationic charge modifying agent. Thesereactions may be represented as follows: ##STR5##

The polyepoxide cross-linking agent thus serves several functions:

1. The polyepoxide cross-links the primary amine groups on thehydrophilic membrane to the primary and/or secondary amine groups on thecationic charge modifying agent;

2. The polyepoxide cross-links the carboxyl groups on the hydrophilicmicroporous membrane to the primary and/or secondary amine groups of thecationic charge modifying agent;

3. The polyepoxide cross-links the primary and/or secondary amines ofthe charge modifying agent to each other.

Due to the necessity for the cross-linking agent to function in such amanner, monoepoxides, e.g. glycidyl ethers, are unsuitable for use inthis invention. Similarly, it is theorized that a polyepoxide offeringgreater than three epoxide groups offers no benefit, and in fact maylimit the coupling reactions of the polyepoxide by steric hindrance.Additionally, the presence of unreacted epoxide groups on the chargemodified microporous membrane may be undesirable in the finished chargemodified membrane.

The amines used in this invention are selected in view of the followingtheoretical considerations. Amines are classified as primary, secondaryor tertiary, according to the number of substituents attached to thenitrogen atom, i.e. according to the number of hydrogens which have beensubstituted: ##STR6##

Epoxide groups will react with primary and secondary amine groupsthrough the free hydrogens. An epoxide group will not react with atertiary amine group since there are no free hydrogens.

Amine groups of all three classes, i.e. primary, secondary or tertiaryare capable of forming hydrogen bonds with water. As a result, amines ofrelatively low molecular weight, i.e. short carbon chain length arequite soluble in water, with border line solubility in water occurringat about 6 carbon atoms per amine group. In the preferred embodiment ofthis invention it is highly desirable that the cationic charge modifyingagent be soluble in water to provide the desired environment forproduction, i.e., elimination of fumes, toxicity, etc.

Amines are basic and generally form salts:

    R.sub.1 NH.sub.2 +H.sup.+ ⃡R.sub.1 NH.sub.3.sup.+

    R.sub.2 NH+H.sup.+ ⃡R.sub.2 NH.sub.2 +

    R.sub.3 N+H.sup.+ ⃡R.sub.3 NH+

The amines are converted into their salts, i.e. charged form, byhydrogen ions and are liberated from their salts by hydroxide ions:##STR7##

It is this latter characteristic, that produces an undesirable reductionin positive surface charge on the microporous membrane (as measured byelectrophoretic mobility or streaming potential), and the correspondingreduction in adsorptive capacity for anionic contaminants that has beennoted when amine charge modified filter media is tested over a series ofincreasing pHs. It would therefore appear that the more basic the aminecharge modifying agent, the higher is the charge modification andadsorptive capacity for contaminants that a filter media, e.g. membrane,will exhibit at a given pH.

Basicity of an amine is defined by measuring the extent to which theamine can accept hydrogen ions from water, with the equilibrium constantfor this reaction being the basicity constant Kb: ##EQU1##

From the literature, we find that aliphatic amines of all three classes,i.e. primary, secondary and tertiary, have Kb's that range from about10⁻³ to 10⁻⁴, and are stronger bases than ammonia. This 10 to 1 range ofKb for aliphatic amines indicates that some amines will be better chargemodifiers (high Kb) than others (low Kb). It is theorized that aromaticamines, which are considerably weaker bases than ammonia, having a Kb of10⁻⁹ or less, are unsuitable as charge modifying agents.

To select from among the aliphatic amines a preferred embodiment on atheoretical basis becomes somewhat more complicated, due to the factthat one is concerned with the basicity of the amine bonded through thepolyepoxide to the microporous membrane. From applicants' testing (seeExamples) it appears as through the basicity of the cross-linkedtetraethylene pentamine (as measured by dye absorption capacity at pH7.0) increases with increased epoxide level, i.e. increasedcross-linking of the amine. This would appear to indicate increasingbasicity (and thus filtration effectiveness) as the extent to which theprimary and secondary amines originally present in the tetraethylenepentamine, are converted to tertiary amines via the reaction with theepoxide.

Preferably, when the two step process for producing the membrane isused, the membrane between the contacting steps is drained for a periodof time sufficient to remove most of the water and chemical compound notabsorbed or adsorbed onto the surface of the membrane. Optionally, themembrane may be transferred directly from the first contacting step tothe second contacting step, although this is less preferred. Theintermediate treatment may also be a restrained drying step.

After the microporous membrane has been contacted with the aqueoussolution(s), it is then dried and cured, preferably in a restrainedcondition to prevent shrinkage.

Drying of the membrane under restraint is described in the assignee'sco-pending U.S. Ser. No. 201,086 to Repetti filed Oct. 27, 1980, nowdefensive publication T-103,601. The entire disclosure of thisapplication is incorporated herein by reference. Generally, any suitablerestraining technique may be used while drying, such as winding themembrane tightly about a drying surface, e.g. a drum. Bi-axial controlis preferred and tensioning the membrane on a stretching frame isconsidered the most preferred. Preferably, the restraint imposed affectsno reduction in dimensions.

Final drying and curing temperatures should be sufficient to dry andcure the polyepoxide-amine, preferably from about 120 C. to 140 C. forminimization of drying times without embrittlement or other detrimentalaffects to the membrane.

The completed membrane may be rolled and stored for use under ambientconditions. It will be understood that the treated membrane may besupplied in any of the usual commercial forms, for example, as discs orpleated cartridges.

In today's processes for producing electronic components, micron andsubmicron sized particles can cause large enough faults in electroniccircuits to cause unacceptably high reject rates, resulting in reducedyields and higher costs. The high purity water required in themanufacture of semi-conductors must contain no dissolved solids and havea resistivity of at least about 18 megohm-cm. In such criticalapplications, it is necessary to minimize the contaminants the filtercontributes as well as maximize the contaminants the filter removes.Therefore, the filter must be flushed out prior to use, e.g. immediatelyafter installation in the process, to remove trace amounts of solubleextractables that might alter resistivity of the water and indirectlyadd dissolved ionics to the water. An excellent measure of filter purityis the time required to flush out the filter to a point at which the 18megohm-cm water suffers no loss in resistivity in passing through thefilter. The cationically charged microporous membrane of this invention,as demonstrated by the Examples herein, has an extremely rapid flush outcharacteristic when compared with the closest prior art charge modifiedmembrane known to applicant, i.e. the membrane described in U.S. Ser.No. 201,366 to Ostreicher et al (ZETAPOR from AMF Cuno) and even thenon-modified membrane. This characteristic makes the charge modifiedmicroporous membrane of this invention particularly suitable for use inthe filtration of high purity water, i.e. at least about 18 megohm-cmresistivity water.

Having now generally described this invention, the same will becomebetter understood by reference to certain specific examples, which areincluded herein for the purposes of illustration only and are notintended to be limiting of the invention, unless so specified.

EXAMPLES

A series of tests were conducted to investigate the characteristics ofthe charge modified membrane of this invention and process for producingsuch membrane. The test procedure was designed to permit a directcomparison with the preferred embodiment described in the aforementionedU.S. Ser. No. 201,366 and to determine the preferred embodiment of theinvention described and claimed herein.

The membrane used for this test series was a single sheet of unmodified0.2 um nylon membrane produced pursuant to the aforementioned Marinaccioet al patent. Each Example group consisted of nine (9) adjacent 13"×16"sheets. In turn, each nine (9) piece group was divided into three (3)piece subgroups "A" and "B" and "C". The "A" group was left untreatedand was used to provide baseline data for "unmodified" or "beforetreatment" membrane. The "B" and "C" groups were subjected to thetreatment modes given in Table I. The "A" and "B" groups were subjectedto the following measurements and tests:

Thickness--4 samples, 47 mm disc

IBP¹ and FAOP² --4 samples, 47 mm disc

Flow--4 samples, 47 mm disc

Dye Adsorption--2 samples 47 mm disc

Flushout--2 samples, 293 mm disc

1. Initial Bubble Point

2. Foam All Over Point

The "C" groups were retained.

The measurement and test procedures utilized were as follows:

Thickness

The dry membrane thickness was measured with a 1/2 inch diameter platendial indicator thickness gauge. Gauge accuracy was ±0.00005 inches(±0.05 mils).

Initial Bubble Point (IBP) and Foam-All-Over-Point (FAOP) Test

A 47 mm diameter disc of the membrane sample is placed in a special testholder which seals the edge of the disc. Above the membrane, anddirectly in contact with its upper face, is a perforated stainless steelsupport screen which prevents the membrane from deforming or rupturingwhen air pressure is applied to its bottom face. Above the membrane andsupport screen, the holder provides a 1/2 inch deep cavity into whichdistilled water is introduced. A regulated air pressure is introducedbelow the membrane and the pressure is increased until a first stream ofair bubbles is emitted by the water-wetted membrane into the quiescentpool of water. The air pressure at which this first stream of airbubbles is emitted is called the Initial Bubble Point (IBP) of thelargest pore in that membrane sample. Once the Initial Bubble Pointpressure has been determined and recorded, the air pressure is furtherincreased until the air flow thru the wetted membrane sample, asmeasured by a flow meter in the line between the regulator and thesample holder, reaches 100 cc/min. The air pressure at this flow rate,is called the Foam-All-Over-Point (FAOP), and is directly proportionalto the mean pore diameter of the sample membrane. In this series oftests, these two parameters (IBP and FAOP) are used to determine if anychange has occurred in the maximum or mean pore size of the membranesample as a result of the charge modifying process utilized.

Flow Rate Test

A 47 mm diameter disc of the membrane sample is placed in a test housingwhich allows pressurized water flow thru the membrane. Prefiltered wateris passed thru the membrane sample at a pressure differential of 5 PSID.A graduate cylinder is used to measure the volume of water passed by themembrane sample in a one minute period. In this series of tests thisparameter is used in conjunction with the IBP and FAOP to determine ifany reduction in pore size or pore blockage has occurred as a result ofthe charge modifying process utilized.

Dye Adsorption Test

A 47 mm diameter disc of the membrane sample is placed in a test housingwhich allows pressurized water flow thru the membrane. The challengesolution consists of distilled water at a pH of 7.0, and Metanil Yellowdye. The dye inlet concentration is adjusted to produce a 76 percenttransmittance at a wavelength of 430 nm, as measured on a Perkin-ElmerModel 295 Spectrophotometer. By means of a peristaltic pump thechallenge solution is flowed thru the membrane sample at a flow rate of28 ml/min. The transmittance of the effluent is measured by passing itthru a constant flow cell in the aforementioned Spectrophotometer. Theeffluent transmittance and pressure drop across the membrane aremeasured and recorded as a function of time. The test is terminated whenthe effluent transmittance increases to 85 percent of the inlettransmittance. In this series of tests, the length of time that it takesto reach the 85 percent transmittance in the effluent is called the"breakthru" time. Since the Metanil Yellow is a low molecular weightanionic dye incapable of being mechanically removed (filtered) by themembrane, this breakthru time is proportional to the cationic adsorptivecapacity of the membrane sample. This test is therefore used todetermine the effectiveness of the charge modification technique.

Flushout Test

A 293 mm diameter disc of the membrane sample is installed in a testhousing which allows pressurized water flow thru the membrane.Prefiltered and deionized 18 megohm-cm water is flowed thru the membranesample at a flow rate of 2.5 GPM. The effluent resistivity is constantlymonitored. The length of time that is required for the membrane effluentto reach a resistivity of 18 megohms-cm is determined and recorded.

The results obtained from the testing of the various treatment modes(see Table I and III) are statistically summarized in Table II and IV.From these results, the following conclusions can be made:

1. The preferred embodiment charge modification treatment of the priorart (Example #1) gives excellent charge modification (as determined bydye adsorption) but results in unacceptably long flushout times, evenwhen using 18 megohm-cm water to produce the membrane.

2. In any of the tested treatment modes (Examples 2 thru 15 and 18 thru22) the charge modifying process of this invention exhibits good toexcellent charge modification characteristics (as measured by dyeadsorption) and extremely short flushout times. These flushout times aresignificantly less than that of the prior art (Example #1) and, evenmore significant, are less than those of the untreated membrane.

3. None of the treatment of this invention produce any pore sizereduction or pore blockage.

4. In terms of optimized flushout characteristics, Example 9 wouldappear to represent the preferred embodiment.

A 293 mm diameter disc of Example 9B was placed in contact with Meridencity water (resistivity approximately 9000 ohm-cm) and then subjected toa flushout test. In this case, the flushout time was 30.23 minutes ascompared to the 2.27 minute flushout of the original 9B sample. Thisdemonstrates the need to use low ionic content water if one desires amembrane having rapid flushout characteristics.

5. In terms of maximized charge modification, Example 21, would appearto represent the preferred embodiment.

6. Although aqueous solutions are preferred, Example 22 shows that othersolutions, e.g. non-aqueous solutions, may be utilized with no adverseeffect on the membrane performance characteristics.

                                      TABLE I                                     __________________________________________________________________________    TREATMENT MODES                                                               EXAMPLE                INTERMEDIATE                                           NO.    1ST STAGE TREATMENT                                                                           TREATMENT   2ND STAGE TREATMENT                                                                           POST                       __________________________________________________________________________                                                       TREATMENT                  1      4308 Resin - 2 wt % sol.                                                                      Air dry and drain for                                                                     0.03 wt. % sol. Pentamine                                                                     Drain and Stretch Dry             pH adjusted to 10.5                                                                           3 minutes                                              2      1.5 wt. % DGE in 80/20                                                                        Air dry and drain for                                                                     0.5 wt. % Pentamine in                                                                        "                                 Water - Ethanol Solution                                                                      3 minutes   Water Solution                             3      2.0 wt % DGE in 80/20                                                                         Air dry and drain for                                                                     0.5 wt. % Pentamine in                                                                        "                                 Water - Ethanol Solution                                                                      3 minutes   Water Solution                             4      2.5 wt % DGE in 80/20                                                                         Air dry and drain for                                                                     0.5 wt. % Pentamine in                                                                        "                                 Water - Ethanol Solution                                                                      3 minutes   Water Solution                             5      1.0 wt % DGE in 80/20                                                                         Air dry and drain for                                                                     0.25 wt % Pentamine in                                                                        "                                 Water - Ethanol Solution                                                                      3 minutes   Water Solution                             6      4.0 wt % DGE in 80/20                                                                         Air dry and drain for                                                                     1.0 wt % Pentamine in                                                                         "                                 Water - Ethanol Solution                                                                      3 minutes   Water Solution                             7      0.5 wt % Pentamine in                                                                         Air dry and drain for                                                                     1.5 wt % DGE in 80/20                                                                         "                                 Water Solution  3 minutes   Water - Ethanol Solution                   8      0.5 wt % Pentamine in                                                                         Air dry and drain for                                                                     2.0 wt % DGE in 80/20                                                                         "                                 Water Solution  3 minutes   Water - Ethanol Solution                   9      0.5 wt % Pentamine in                                                                         Air dry and drain for                                                                     2.5 wt % DGE in 80/20                                                                         "                                 Water Solution  3 minutes   Water - Ethanol Solution                   10     1.0 wt % Pentamine in                                                                         Air dry and drain for                                                                     4.0 wt % DGE in 80/20                                                                         "                                 Water Solution  3 minutes   Water - Ethanol Solution                   11     0.25 wt % Pentamine in                                                                        Air dry and drain for                                                                     1.0 wt % DGE in 80/20                                                                         "                                 Water Solution  3 minutes   Water - Ethanol Solution                   12     2.0 wt % DGE in 80/20                                                                         Directly transfer into                                                                    0.5 wt % Pentamine in                                                                         "                                 Water - Ethanol Solution                                                                      2nd stage treatment                                                                       Water Solution                             13     0.5 wt % Pentamine in                                                                         Directly transfer into                                                                    2.0 wt % DGE in 80/20                                                                         "                                 Water Solution  2nd stage treatment                                                                       Water - Ethanol Solution                   14     2.0 wt % DGE in 80/20                                                                         Drain and Stretch Dry                                                                     0.5 wt % Pentamine in                                                                         "                                 Water - Ethanol Solution                                                                      in Air      Water Solution                             15     0.5 wt % Pentamine in                                                                         Drain and Stretch Dry                                                                     2.0 wt. % DGE in 80/20                                                                        "                                 Water Solution  in Air      Water - Ethanol Solution                   16     None            None        Soak in high purity water                                                                     "                          __________________________________________________________________________     4308 Resin is Hercules, Inc. R4308 resin polyamido-polyamine epichlorydri     resin.                                                                        DGE is 1,4butanediol diglycidyl ether.                                        Pentamine is tetraethylene pentamine.                                    

                                      TABLE II                                    __________________________________________________________________________    SUMMARY OF TEST RESULTS                                                                      INITIAL                                                                              FOAM-                                                          THICKNESS                                                                             BUBBLE ALL-OVER                                                                              FLOW RATE                                                                             DYE ADSORPTION                                                                            FLUSHOUT TO 18              EXAMPLE                                                                              (MILS)  POINT (PSI)                                                                          POINT (PSI)                                                                           (ML/MIN.)                                                                             BREAKTHRU (MIN.)                                                                          MEGOHM (MINUTES)            NO.    B.T.                                                                              A.T.                                                                              B.T.                                                                             A.T.                                                                              B.T.                                                                              A.T.                                                                              B.T.                                                                              A.T.                                                                              B.T.  A.T.  B.T.   A.T.                 __________________________________________________________________________    1      4.68                                                                              4.59                                                                              44.9                                                                             45.3                                                                              51.8                                                                              50.9                                                                              75.0                                                                              86.0                                                                              9.9   49.3  6.90   69.57                2      4.71                                                                              4.69                                                                              45.9                                                                             43.4                                                                              51.9                                                                              51.0                                                                              78.3                                                                              82.5                                                                              9.9   26.3  8.67   3.02                 3      4.56                                                                              4.65                                                                              45.6                                                                             45.6                                                                              52.0                                                                              51.0                                                                              80.8                                                                              79.3                                                                              7.9   34.6  7.90   3.88                 4      4.56                                                                              4.76                                                                              44.8                                                                             47.5                                                                              51.8                                                                              53.6                                                                              81.0                                                                              73.8                                                                              7.7   40.0  16.05  4.03                 5      4.61                                                                              4.70                                                                              45.9                                                                             46.8                                                                              52.1                                                                              52.6                                                                              80.3                                                                              77.0                                                                              9.4   16.6  16.58  4.28                 6      4.61                                                                              4.73                                                                              47.6                                                                             45.3                                                                              52.6                                                                              52.5                                                                              76.3                                                                              77.3                                                                              10.2  49.4  8.23   6.48                 7      4.58                                                                              4.64                                                                              45.8                                                                             46.4                                                                              51.9                                                                              52.3                                                                              81.5                                                                              80.5                                                                              9.3   22.5  5.88   1.35                 8      4.58                                                                              4.65                                                                              47.4                                                                             44.8                                                                              52.0                                                                              51.5                                                                              81.5                                                                              80.0                                                                              8.4   26.5  5.18   1.78                 9      4.58                                                                              4.73                                                                              47.1                                                                             44.6                                                                              52.3                                                                              50.0                                                                              83.0                                                                              80.8                                                                              8.8   36.8  7.70   2.27                 10     4.66                                                                              4.76                                                                              44.9                                                                             43.5                                                                              51.8                                                                              51.3                                                                              74.8                                                                              79.8                                                                              8.3   39.2  9.05   9.02                 11     4.58                                                                              4.63                                                                              45.6                                                                             46.0                                                                              52.9                                                                              52.0                                                                              76.5                                                                              81.3                                                                              10.7  21.0  8.10   1.00                 12     4.60                                                                              4.63                                                                              45.3                                                                             45.3                                                                              52.1                                                                              52.5                                                                              76.8                                                                              80.5                                                                              10.0  37.2  8.08   5.60                 13     4.56                                                                              4.74                                                                              45.0                                                                             46.1                                                                              52.6                                                                              52.9                                                                              77.3                                                                              78.0                                                                              7.7   23.9  8.33   1.70                 14     4.59                                                                              5.25                                                                              46.3                                                                             49.0                                                                              52.5                                                                              56.0                                                                              75.0                                                                              83.8                                                                              8.5   37.3  10.33  5.07                 15     4.60                                                                              4.75                                                                              46.3                                                                             46.4                                                                              52.8                                                                              53.5                                                                              74.8                                                                              76.0                                                                              5.5   26.3  7.33   4.07                 16     4.58                                                                              4.74                                                                              46.6                                                                             47.0                                                                              53.1                                                                              53.5                                                                              76.5                                                                              77.0                                                                              5.2   6.75  8.20   7.65                 __________________________________________________________________________     B.T. is Before Treatment                                                      A.T. is After Treatment                                                  

EXAMPLE 17

Nylon membrane was treated, in the preferred mode disclosed in copendingU.S. Ser. No. 201,366, as follows:

1st stage--Hercules R 4308 PAE (polyamido polyamine epichlorhydrin)Resin 2.0 wt. % solution adjusted to pH 10.5 with NaOH.

2nd stage--Tetraethylene Pentamine, 0.03 wt. % solution.

The treated nylon was allowed to drain and then stretch dried. Thetreated membrane, and an untreated control, were tested by flushing outa 293 mm disc at 2 GPM with 18 megohm-cm influent resistivity water. Thetest results were as follows:

    ______________________________________                                                                   Final Effluent                                     Sample         Flushout Time                                                                             Resistivity                                        ______________________________________                                        Untreated Control                                                                             5.0 minutes                                                                               18 megohm                                         Treated Sample 35.0 minutes                                                                              7.8 megohm                                         ______________________________________                                    

Identical to Example 17A, except pH of 1st stage treatment was notadjusted with NaOH. The Test results were as follows:

    ______________________________________                                                                   Final Effluent                                     Sample         Flushout Time                                                                             Resistivity                                        ______________________________________                                        Untreated Control                                                                             5.0 minutes                                                                              18 megohm                                          Untreated Control                                                                             6.0 minutes                                                                              18 megohm                                          Treated Sample 2l.0 minutes                                                                              6.1 megohm                                         Treated Sample 25.0 minutes                                                                              7.75 megohm                                        ______________________________________                                    

A washed and stretch dried nylon microporous membrane was treated, byimmersion in a 2.0 weight percent solids 1,4-butanediol diglycid 1 ethersolution prepared with an 80-20 mixture of 18 megohm-cm D.I. water andhigh purity ethanol. This solution exhibited a pH of 6.1-6.4. Themembrane was removed from this solution and allowed to drain forapproximately one minute.

The membrane is then immersed in a 0.5 weight percent solids solution oftetraethylene pentamine prepared with 18 megohm-cm D.I. water. Thissolution exhibited a pH of 11.2-11.4. The membrane is removed from thissolution and allowed to drain for approximately one (1) minute.

The treated membrane is then stretch dried at 130° C. for five minutes.

The test results are as follows:

    ______________________________________                                        Sample    Flushout Time                                                                             Final Effluent Resistivity                              ______________________________________                                        Treated   3.25 minutes                                                                              18 megohm-cm                                            ______________________________________                                    

                                      TABLE III                                   __________________________________________________________________________    TREATMENT MODES                                                               EXAMPLES 18-22                                                                EXAMPLE                                                                              1ST STAGE    INTERMEDIATE                                                                              2ND STAGE                                     NO.    TREATMENT    TREATMENT   TREATMENT  POST TREATMENT                     __________________________________________________________________________    18     2.0 wt % EGDGE in                                                                          Air dry and drain for 3                                                                   0.5 wt % Pentamine in                                                                    Drain and Stretch Dry                     Water Solution                                                                             minutes     Water Solution                                19     2.0 wt % EGDGE and                                                                         Drain and                                                        0.5 wt % Pentamine                                                                         stretch dry                                                      in water solution                                                      20     2.0 wt % DGE in 80/20                                                                      Air dry and drain for 3                                                                   0.5 wt % Pentamine in                                                                    Drain and Stretch Dry                     Water - Ethanol Solution                                                                   minutes     Water Solution                                21     2.0 wt % DGE and                                                                           Drain and                                                        0.5 wt % Pentamine in                                                                      stretch dry                                                      80/20 Water - Ethanol Sol.                                             22     2.0 wt % DGE in                                                                            Drain and dry at 150° C.                                                           0.5 wt % Pentamine in                                                                    Drain and stretch dry                     Acetone      for 1 minute                                                                              Water Solution                                __________________________________________________________________________     EGDGE is 1,2  ethanediol diglycidyl ether                                     DGE is 1,4  butanediol diglycidyl ether                                       Pentamine is tetraethylene pentamine                                     

                                      TABLE IV                                    __________________________________________________________________________    EXAMPLES 18-22                                                                SUMMARY OF TEST RESULTS                                                                                FOAM            DYE                                         THICKNESS                                                                             INITIAL BUBBLE                                                                          ALL OVER                                                                              FLOW RATE                                                                             ADSORPTION                                                                             FLUSHOUT TO 18              EXAMPLE                                                                              (MILS)  POINT (PSI)                                                                             POINT   (ML/MIN)                                                                              BREAKTHRU                                                                              MEGOHM (MIN.)               NO.    B.T A.T BT   AT   BT  AT  BT  AT  BT   AT  BT    AT                    __________________________________________________________________________    18     --  --  49.0 46.0 53.5                                                                              56.0                                                                              75  69  --   19.5                                                                              ˜10                                                                           3.50                  19     --  --  49.0 48.0 53.5                                                                              48.5                                                                              75  71  --   73.5                                                                              ˜10                                                                           5.52                  20     --  --  49.0 49.5 53.5                                                                              51.0                                                                              75  67  --   31.5                                                                              ˜10                                                                           3.31                  21     --  --  49.0 49.0 53.5                                                                              54.0                                                                              75  69  --   96.0                                                                              ˜10                                                                           6.27                  22     --  --  --   --   --  --  --  --  5.44 39.28                                                                             --    5.0                   __________________________________________________________________________     B.T. is Before Treatment                                                      A.T. is After Treatment                                                  

What is claimed is:
 1. A skinless cationic charge modified hydrophilicmicroporous membrane comprising a skinless hydrophilic organic polymericmicroporous filter membrane having an internal microstructure throughoutsaid membrane and a charge modifying amount of a cationic chargemodifying agent bonded to substantially all of the membranemicrostructure without substantial pore size reduction or pore blockagethrough an aliphatic polyepoxide cross-linking agent having a molecularweight of less than about 500, wherein the charge modifying agent isselected from the group consisting of:(i) aliphatic amines which arepolyamines having at least one primary amine or at least two secondaryamines; and (ii) aliphatic amines having at least one secondary amineand a carboxyl or hydroxyl substituent.
 2. The microporous membrane ofclaim 1, wherein the hydrophilic organic polymeric microporous membraneis nylon.
 3. The microporous membrane of claim 1, wherein thehydrophilic organic polymeric microporous membrane is polyhexamethyleneadipamide.
 4. The microporous membrane of claim 1 or 2, wherein thecharge modifying agent is an amine of the formula: ##STR8## wherein R₁and R₂ are alkyl of 1 to 4 carbon atoms and x is an integer from 0 to 4.5. The microporous membrane of claim 4, wherein R₁ and R₂ are ethyl. 6.The microporous membrane of claim 4, wherein the amine is tetraethylenepentamine of the formula: ##STR9##
 7. The microporous membrane of claim1 or 2, wherein the polyepoxide is a di- or tri-epoxide.
 8. Themicroporous membrane of claim 7, wherein the polyepoxide has a molecularweight of from about 146 to about
 300. 9. The microporous membrane ofclaim 8, wherein the polyepoxide has the formula: ##STR10## wherein R isan alkyl of 1 to 6 carbon atoms and n is an integer from 2 to
 3. 10. Themicroporous membrane of claim 6, wherein the polyepoxide is1,4-butanediol diglycidyl ether of the formula: ##STR11##
 11. A processfor cationically charge modifying a hydrophilic organic polymericmicroporous membrane comprising applying to the membrane a chargemodifying amount of a cationic charge modifying agent which is bonded tothe membrane structure through an aliphatic polyepoxide cross-linkingagent having a molecular weight of less than 500, wherein the chargemodifying agent is selected from the group consisting of:(i) aliphaticamines having at least one primary amine or at least two secondaryamines; and (ii) aliphatic amines having at least one secondary amineand a carboxyl or hydroxyl substituent.
 12. A process for cationicallycharge modifying a hydrophilic organic polymeric microporous membranecomprising:(a) contacting the membrane with an aqueous solution of acationic charge modifying agent, wherein the charge modifying agent isselected from the group consisting of:(i) aliphatic amines having atleast one primary amine or at least two secondary amines; and (ii)aliphatic amines having at least one secondary amine and a carboxyl orhydroxyl substituent, (b) contacting the membrane with an aqueoussolution of an aliphatic polyepoxide cross-linking agent having amolecular weight of less than about
 500. 13. The process of claim 12,wherein the contacting step (a) precedes the contacting step (b). 14.The process of claim 12, wherein the contacting step (b) precedes thecontacting step (a).
 15. The process of claim 12, wherein the contactingsteps (a) and (b) are combined and the membrane is contacted with anaqueous solution of a mixture of the charge modifying agent and thepolyepoxide.
 16. The process of claim 12, wherein the aqueous solutionsof charge modifying agent and polyepoxide are produced with watersubstantially free of ionic content.
 17. The process of claim 16,wherein the water has a resistivity of at least about 250,000 ohm-cm.18. The process of claim 12, wherein the aqueous solution of chargemodifying agent contains at least about 0.25% amine by weight of thesolution.
 19. The process of claim 12, wherein the aqueous solution ofpolyepoxide contains at least 1% polyepoxide by weight of the solution.20. The process of claim 18, wherein the aqueous solution of the chargemodifying agent contains up to 1% amine by weight of the solution. 21.The process of claim 19, wherein the aqueous solution of polyepoxidecontains up to about 4% polyepoxide by weight of the solution.
 22. Theprocess of claim 12, wherein the weight ratio of polyepoxide to chargemodifying agent is from about 2:1 to about 10:1.
 23. The process ofclaim 22, wherein the weight ratio is from about 3:1 to about 5:1. 24.The process of claim 11 or 12, wherein the microporous membrane isnylon.
 25. The process of claim 11 or 12, wherein the microporousmembrane is polyhexamethylene adipamide.
 26. The process of claim 11 or12, wherein the charge modifying agent is an amine of the formula:##STR12## wherein R¹ and R² are alkyl of 1 to 4 carbon atoms and x is aninteger from 0 to
 4. 27. The process of claim 26, wherein R₁ and R₂ areethyl.
 28. The process of claim 11 or 12, wherein the amine istetraethylene pentamine of the formula: ##STR13##
 29. The process ofclaim 11 or 12, wherein the polyepoxide is a di- or tri-epoxide.
 30. Theprocess of claim 29, wherein the polyepoxide has a molecular weight offrom about 146 to about
 300. 31. The process of claim 11 or 12, whereinthe polyepoxide has the formula: ##STR14## wherein R is an alkyl of 1 to6 carbon atoms and n is an integer from 2 to
 5. 32. The process of claim28, wherein the polyepoxide is 1,4-butanediol diglycidyl ether of theformula: ##STR15##
 33. A process for filtration of fluids comprisingpassing the fluid through a filter media comprising the microporousmembrane of claim
 1. 34. The process of claim 33, wherein the fluid iswater having at least about 18 megohm-cm resistivity.